1. Field of the Invention
This invention relates generally to filtration material, and more particularly to a filtration media comprising two types of fibers blended and holding an electrostatic charge.
2. Description of the Related Art
It is known in the filtration art that various kinds of fibers can be formed into a web or other structure with small, tortuous paths through which gases, such as air, are passed to remove particulate matter from the gas stream. The particulate matter in the gas flowing through the paths in the web is retained on the upstream side of the web or within the paths of the web due to the size of the particles being larger than the paths' diameters.
Furthermore, it is known to electrostatically charge various blends of fibers to further retain particulate matter through electrostatic attraction between the fibers and the particles. Such blends are shown in U.S. Pat. No. 6,328,788 to Auger, U.S. Pat. No. 4,798,850 to Brown, U.S. Pat. No. 5,470,485 to Morweiser, et al., and U.S. Pat. No. 5,792,242 to Haskett, all of which are incorporated herein by reference.
The Brown patent discloses a blend of two types of fibers: polyolefin and a substituted additional polymer. The first fiber in Brown is preferably polypropylene and the second is modacrylic. The preferred range of surface area ratios (polypropylene to modacrylic) in Brown is 30:70 to 80:20, and more preferably 40:60 to 70:30.
Furthermore, the polypropylene fibers used in the Brown patent are clean, which is described in the patent as follows: “By ‘clean’ we mean that the fibre has no coating of lubricant or anti-static agent, or that any such coating was removed before blending, for example, by scouring with a non-ionic detergent.” Thus, the fibers of the Brown patent are without lubricant or anti-static agent, typically by being manufactured with a finish and subsequently removing the finish by scouring. However, the step of cleaning fibers adds cost to the finished filtration product and may deleteriously affect the fibers.
U.S. Pat. No. 6,623,548 to Gordon discloses an electrostatically charged filter material comprising a blend of polypropylene fibers and (a) halogen free acrylic fibers or (b) polyvinyl chloride (PVC) fibers or (c) a mixture of PVC and halogen free acrylic fibers. In this patent, the fibers are also cleaned to remove antistatic and lubricant agents. This is described in the Gordon patent as follows: “In practise, polymer fibres produced commercially have spin finishes, and thus it is necessary to substantially remove lubricants and anti-static agents from the fibres before, during or [ ] after the blending of said fibres.”
The prior art filtration materials provide sufficient filtration for many environments. However, where the electrostatic charge must be retained for long periods or where the penetration through the filtration material must be below a particular percentage, the prior art does not suffice, or is prohibitively expensive to manufacture due to the need for the scouring step. Therefore, the need exists for a filtration material that provides the needed performance at a feasible cost to manufacture.